This page looks at the reaction of the carbon-carbon double bond in alkenes such as ethene with halogens such as chlorine, bromine and iodine. This is called halogenation. Reactions where the chlorine or bromine are in solution (for example, "bromine water") are slightly more complicated and are treated separately at the end.
Simple reactions involving halogens
In each case, we will look at ethene as typical of all of the alkenes. There are no complications as far as the basic facts are concerned as the alkenes get bigger.
Ethene and fluorine
Ethene reacts explosively with fluorine to give carbon and hydrogen fluoride gas. This isn't a useful reaction, and you aren't likely to need it for exam purposes in the UK at this level (A level or equivalent).
Ethene and chlorine or bromine or iodine
In each case you get an addition reaction. For example, bromine adds to give 1,2-dibromoethane.
The reaction with bromine happens at room temperature. If you have a gaseous alkene like ethene, you can bubble it through either pure liquid bromine or a solution of bromine in an organic solvent like tetrachloromethane. The reddish-brown bromine is decolourized as it reacts with the alkene. A liquid alkene (like cyclohexene) can be shaken with liquid bromine or its solution in tetrachloromethane. Chlorine reacts faster than bromine, but the chemistry is similar. Iodine reacts much, much more slowly, but again the chemistry is similar. You are much more likely to meet the bromine case than either of these.
Alkenes decolourize bromine water.
If you shake an alkene with bromine water (or bubble a gaseous alkene through bromine water), the solution becomes colorless. This is complicated by the fact that the major product isn't 1,2-dibromoethane. The water also gets involved in the reaction, and most of the product is 2-bromoethanol.
However, there will still be some 1,2-dibromoethane formed, so at this sort of level you can probably get away with quoting the simpler equation:
Jim Clark (Chemguide.co.uk)